Chemical Activation and Refining of Southern Pine Kraft Fibers

ABSTRACT

A method for alteration of the morphology of cellulose fibers, particularly softwood fibers, by (a) subjecting the fibers to a metal ion-activated peroxide treatment carried out at a pH of between about 1 and about 9, preferably between 3 and 7, and (b) subjecting the treated fibers to a refining treatment thereby converts SW fibers to 
     HW-like fibers in many respects. The metal ion-activated peroxide treatment has been noted to act on pulp cellulose and hemi-cellulose, causing oxidation and oxidative degradation of cellulose fibers. The chemical treatment of the pulp, taken alone, is not sufficient to attain the desired modification of the morphology of the fibers, however, subsequent refining or like mechanical treatment of the chemically-treated fibers to achieve a given degree of refinement of the fibers requires dramatically less refining energy to achieve a desired end point of refinement and to impart other desirable properties to the pulp. A pulp of modified SW fibers and a mixture of HW fibers and modified HW fibers are disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

This invention relates to papermaking and particularly to the treatmentof cellulosic material preparatory to use of the treated material tomanufacture paper web material.

As is well known in the art, paper is commonly formed from wood.Generally, the industry divides wood used in papermaking into twocategories; namely hardwoods and softwoods. Softwood fibers (tracheids)come from needle-bearing conifer trees such as pine, spruce, alpine fir,and Douglas fir. Hardwood fibers are derived from deciduous trees ofvarious varieties.

Among the distinguishing differences between hardwood (HW) fibers andsoftwood (SW) fibers are(a) the length of the individual cellulosicfibers of the wood, (b) the coarseness of the fibers, and (c) thestiffness or collapsibility of the fibers.

The morphology of softwood fibers, tends to limit the potential uses ofthe papers producible from such fibers. “Paper” as used herein includeswebs or sheets without limitation as to the size or basis weight of theweb or sheet. For example, either HW or SW paper may be employed as“bleached board” (useful in containers for consumer products, forexample) or as “container board” or “liner board” (useful in corrugatedboxes, for example). Printability of a paper is a major considerationwith respect to the end use of the paper. SW fibers are notoriouslyproblematic as respects the printability of the paper produced fromthese fibers in that SW fiber papers tend to be inordinately porous,stiff, and must be treated specially to obtain a paper surface which issuitably printable.

It is well known in the art that HW and SW must be subjected to specifictreatments for converting the wood into a fibrous slurry employed in theformation of a paper web. Softwoods are more plentiful and are morereadily replaceable, as by tree farming. Softwoods in general are lesscostly. Thus, it is desirable that SW fibers be substituted for HWfibers wherever possible in papermaking. Southern pine, or mixtures ofhardwoods and softwoods, are commonly examined as possible substitutesfor end products which have heretofore been manufactured usinghardwoods.

Heretofore, in attempts to utilize SW fibers in printable paper, it hasbeen proposed to treat the pulped fibers with hydrolytic enzymes.Refining of the enzyme-treated fibers to alter their size, shape, degreeof fibrillation, etc., have been employed. Enzyme treatments suffer fromsensitivities of the enzyme to process conditions, and a tendency tobecome inactivated and/or to be carried forward into the papermakingequipment. The lack of cost-effectiveness has also been a long-standingissue.

Chemical treatments, such as hydrogen peroxide treatment, are commonlycarried out under alkaline conditions for bleaching or brightening ofwood pulps. This condition that is maximized for bleaching, usually doesnot correlate with the best conditions for maximum oxidation.

Smoothness and Formation are measures of, among other things, theprintability of the paper. “Formation”, as used as a papercharacteristic usually, and herein, is a synonym for relative uniformityover a scale of some distance, e.g., 5 to 20 mm. Formation may be judgedby viewing it with light from the back and other means. Both smoothnessand formation are affected, among other things, fiber length, morphologyand collapsibility.

BRIEF SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, it has beenfound that alteration of the morphology of cellulose fibers,particularly softwood fibers, by (a) subjecting the fibers to a metalion-activated peroxide treatment carried out at a pH of between about 1and about 9, preferably between 3 and 7, and (b) subjecting the treatedfibers to a refilling treatment converts SW fibers to HW-like fibers inmany respects. The metal ion-activated peroxide treatment has been notedto act on pulp cellulose and hemi-cellulose, causing oxidation andoxidative degradation of cellulose fibers. The chemical treatment of thepulp, taken alone, is not sufficient to attain the desired modificationof the morphology of the fibers, however, subsequent refining or likemechanical treatment of the chemically-treated fibers to achieve a givendegree of refinement of the fibers requires dramatically less refiningenergy, e.g., between about 30 and 50% less energy to achieve a desiredend point of refinement. The pulp treated in accordance with the presentinvention demonstrates substantially reduced fiber length or fiberlength distribution, thereby enabling better uniformity of paper sheet(web) structure as measured by formation or texture. Moreover, thetreated fibers are more collapsible during sheet consolidation andresult in significantly improved paper surface properties such assmoothness. In these respects, SW fibers treated in accordance with thepresent invention are substantially functionally equivalent to HW fibersin regards to their usefulness in papermaking. The treatment of thepresent invention may be applied to wood chemical pulps (or pulpmixtures)having various processing histories such as pulping, bleachingor acid hydrolysis, or other combinations of processing of wood intopulp suitable for infeed to a papermaking machine.

In one embodiment, the present invention may be applied to pulp whichhas already been subjected to refining, chemical treatment, enzymetreatment, microfibrilltion, and/or acid hydrolysis, for example, toincrease the pulp freeness or improve drainage during the papermakingprocess and/or to reduce the cellulose particles suspension viscosityand improving flow characteristic.

In a further embodiment, the advantages of the present invention may beachieved employing a hypochlorite treatment at pH 3-9, preferably, pH3-8 and employing hypochlorous acid as the dominate active agent,followed by subsequent refining of the treated pulp.

Moreover, either the metal ion-activated peroxide or the hypochlorousacid treatment may be applied alone to refined fibers for increasedfreeness/drainage, or on micro-fibrillated cellulose materials forreduced suspension viscosity. Further, either embodiment may be employedas a means for controlling the viscosity of a pulp suspension at any ofvarious locations between the initial digestion of the cellulosematerial to and including the feeding of the pulp suspension into apapermaking machine. This latter aspect of the present invention isapplicable in the dissolution of pulp for viscose production, forexample. In certain stances, the beneficial effects of the presentinvention are exhibited in the calendaring of a paper web or sheetformed from treated SW fibers or combinations of HW fibers and treatedSW fibers.

In a still further embodiment, the present invention may be combinedwith a fiber fractionation process for the treatment of specific fiberfractions.

Paper produced employing pulp treated in accordance with the presentinvention exhibits tear strengths at HW levels, with little materialdeterioration of tensile strength. Improved bonding of the fibers withinthe sheet is also provided due to enhanced freeness.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The above-mentioned features of the invention will become more clearlyunderstood from the following detailed description of the invention readtogether with the drawings in which:

FIG. 1 is a graph depicting the energy savings attributable to thepresent invention when refining Southern Pine pulp;

FIG. 2 is a graph depicting fiber length reduction achieved whentreating Southern Pine pulp in accordance with the present invention;

FIG. 3 is a graph depicting the shifting of fiber length distributionbetween treated and untreated softwood pulp in accordance with thepresent invention;

FIG. 4 is a microphotograph depicting untreated pine fibers;

FIG. 5, is a microphotograph depicting pine fibers treated in accordancewith the present invention;

FIG. 6 is a graph depicting the relationship of bulk vs. smoothness ofhardwood pulp, untreated pine pulp and treated pine pulp;

FIG. 7 is a graph depicting the relationship of bulk vs. freeness of thepulps depicted in FIG. 6;

FIG. 8 is a graph depicting the relationship of tear vs. freeness of thepulps depicted in FIG. 6;

FIG. 9 is a graph depicting bulk and smoothness relationship ofuntreated hardwood pulp, untreated pine pulp, and various mixtures ofhardwood and softwood pulps;

FIG. 10 is a graph depicting the fiber length reduction of untreatedpine pulp and pulp treated in accordance with the present invention,employing low intensity disc refining;

FIG. 11 is a graph depicting the energy savings associated with discrefining employed as a component of the present invention whenprocessing treated and untreated pine pulp; and

FIG. 12 is a graph depicting the relationship between fiber lengthreduction and the energy employed in refining untreated pulp and pulptreated in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided a method for the transformation of softwood fibers,particularly Southern pine fibers, into hardwood-like fibers. The methodemploys the steps of (a) subjecting a SW pulp containing cellulose andhemicellulose, to a solution containing a transitional metal ion and aperoxide at a pH of between about 1 and 9 for a time sufficient tooxidize a substantial portion of the cellulose/hemi-cellulose and tooxidatively degrade the cellulose fibers, and (b) subjecting the treatedpulp to a refining operation. The pulp thus treated, when formed into aweb on a papermaking machine exhibits many hardwood-like properties suchas overall formability into a web having surface properties like websformed from hardwood fibers employing conventional papermakingtechniques.

In one embodiment of the present invention, softwood fibers obtainedfrom coniferous trees, and particularly Southern pine trees, areconverted into a pulp employing the kraft process in which the fibersare treated in a heated alkaline solution to substantially separate thefibers from their lignin binder, as is well known in the art. WhereasSouthern pine fibers are particularly suitable for treatment employingthe present invention, it is recognized that fibers from otherconiferous trees may be employed. Further, the present invention may beadvantageously employed with mixtures of SW and HW fibers, for examplemixtures containing between about 50% and 90% by weight of SW pulp andbetween about 10% and 50% HW pulp.

The SW pulp or mixture of SW and HW pulps, prior to treatment thereofemploying the present invention, may comprise pulp which has notundergone any conventional treatment of the pulp subsequent to thedigestion step. However, the present invention is useful in treatingpulps which, subsequent to digestion, have undergone substantially anyof the commonly employed treatments of pulp such as an acid hydrolysisfor removal of hexauronic acid, oxidation/bleaching employing oxygenand/or peroxide, or ozone, on the pulp and/or mechanical treatment ofthe pulp, ie., refining. In the most commonly contemplated process, thepulp or mixture of pulps, to be subjected to the method of the presentinvention will be a pulp(s) which has been digested and at least washedto remove black liquor.

In accordance with one aspect of the present invention, the pulpsolution, at a temperature of between about 40 and 120 degrees C., issubjected to a solution of a transitional metal-activated peroxide forbetween about 10 and 600 minutes. In general, a higher treatmenttemperature will require less residence time, and vice versa. It ispreferable that the treatment be done at 70-79 degrees C., with aresidence time between 30-180 minutes. The treatment (either continuousor batch) can be carried out in a bleach tower, high-density tower,re-pulper tanks, or any suitable vessel with sufficient mixing andresidence time.

In a preferred embodiment, and contrary to the conventional peroxidetreatment of pulp wherein transitional metal ions are avoided oreliminated to avoid pulp damage or degradation by hydroxyl radicals, thetreatment solution of the present invention, includes between about 0.2%and about 5% by wt. hydrogen peroxide and between about 0.002% and about0.1% of a transitional metal ions, based on pulp. Iron (III) salts suchas ferric chloride, or iron (II) salts such as ferrous sulfate andferrous chloride, are especially useful as a source of the metal ions.Other metal ions, such as copper (II), cobalt(II) may be employed. Inany event, as noted, only a trace of the transitional metal ions isrequired to achieve the advantageous results of the present invention,preferably between about 0.002% and about 0.01% of the metal ion.

Further contrary to conventional peroxide treatment of pulp wherein theperoxide treatment is carried out with the pulp at a very high pH forbleaching, in the present invention, the pulp treatment is carried outat a pH of between about 1 and about 9, preferably a pH between about 2and 7.

Subjection of softwood pulp to the solution of the present invention ata temperature between about 40 C. and about 120 C. and at a pH betweenabout 1 and about 9, has been found to cause oxidation and oxidativepulp degradation of the long, stiff and coarse kraft fibers. Thischemical treatment of the fibers is followed by a mechanical treatmentof the treated pulp, e.g., refining employing a conventional discrefiner, to cause fiber morphology change and paper property enhancementwith respect to hardwood pulps. It will be understood by one skilled inthe art that other mechanical treatment devices which provide equivalentrefining of the pulp fibers may be employed.

Bleached southern pine Kraft pulp from International Paper-Augusta millwas treated at pH 4 with 1% hydrogen peroxide as based on pulp, with0.01% Fe added as with ferric chloride. The treatment was conducted atthe temperature of 80° C. for 1 hour. Both the treated and the control(untreated) pine pulps were refined with a PFI refiner. The data on PFIfreeness and average fiber length are shown in Table I

TABLE I PFI Revolutions 0 Rev. 2000 Revs. 4000 Revs. 6000 Revs ControlFreeness  739 CSF  675 CSF  522 CSF  481 CSF Southern Pine Average Fiber2.50 mm 2.47 mm 2.47 mm 2.42 mm Length, L(L) Treated Freeness  746 CSF 524 CSF  364 CSF — Southern Pine Average Fiber 2.37 mm 1.84 mm 1.64 mm— Length, L(L)

As shown in FIG. 1, the results of refining revolution (indication ofrefining energy) vs. freeness development show that iron catalyzedhydrogen peroxide treatment of pulp enhances pulp refining considerably,resulting in substantial energy savings for reading the same freenesslevel.

FIG. 2 shows the fiber length reduction (length-weighted average) byrefining and indicates that, with catalyzed hydrogen peroxide treatmentbefore refining, the fiber length is substantially reduced after beingsubsequently refined. While for comparison, the untreated pulp (control)showed little fiber length reduction by PFI refining.

FIG. 3 further illustrates the fiber length reduction as shown in FIG.2. In FIG. 3, there is demonstrated the fiber length distributioncurves, with the treated vs. the untreated (control) southern pine, atthe same refining. As seen, the treatment caused a significant shift offiber length to shorter range than the control.

EXAMPLE 2

Bleached southern pine as employed in Example 1 was treated with 1%hydrogen peroxide based on pulp at pH 4, with 0.006% FE(II) as fromferrous sulfate. The treatment was carried out at the temperature of 70°C. for 1 hour. The treated pulp and control were PFI refined as inExample 1. TAPPI hand sheets were then made from these pulps.

To illustrate fiber morphology (beyond fiber length distributions) andfiber collapsibility, SEM (scanning electron microscopy) images weremade of the hand sheet surface of treated vs. the control (untreated)softwood pulps, compared at 4000 Revs of PFI refining. Thesemicrophotographs are depicted in FIGS. 4 (untreated) (control) and 5(treated) and demonstrate that the treated pine fibers are much morecollapsed, or flattened, as compared to the fiber of the control. Thecollapsed and flattened fibers are desirable for making paper orpaperboard with superior surface and printing properties. Some broken orcut fibers (fiber ends) can also be seen from the SEM of treated handsheet, indicating fiber shortening.

EXAMPLE 3

Bleached southern pine pulp was treated with 1% hydrogen peroxidecatalyzed by 0.006% Fe(II) at pH 4 as in the Example 2 above. Thetreated pulps were PFI refined, and made into hand sheets for paperphysical property evaluations. Results are shown in Table II.

TABLE II Tear Basis Factor Extensional Weight, Bulk,  Sheffield 100 *gf/ Stiffness, g/m2 cc/g Smoothness g/m2 lbs/in. Treated Pine Pulp730CSF 151.9 1.90 375.6 190.9 2960 (Unrefined) 556 CSF 155.2 1.34 165.3111.9 4780 421 CSF 154.4 1.36 127.2 103.4 5050 304 CSF 155.2 1.26 129.798.1 5210 Control Pine Pulp 740CSF 162.4 1.91 380 270.9 3490 (Unrefined)661 CSF 155.6 1.40 249.6 193.6 4020 625 CSF 159.9 1.35 185.3 188.7 4340569 CSF 158.5 1.31 191.6 167.4 4540 443 CSF 155.9 1.27 157.8 170.2 4340Bleached Hardwood Pulp 615 CSF 166 1.88 333 52.3 2040 584 CSF 163.1 1.64268.6 87.9 2520 544 CSF 164.9 1.53 224.4 100 2840 507 CSF 161.0 1.40175.2 112.6 3030 462 CSF 160.5 1.36 142.2 126.9 3010 427 CSF 162.8 1.31127.8 107.8 3480 362 CSF 163.9 1.273 89 123.6 3320

From this table, it is noted that the treated pine, after refined to˜560 CSF or lower freeness (to shorten the fibers also), show improvedbulk-smoothness. This is also shown in FIG. 6. FIG. 7 depicts the bulkat given freeness, which suggests the advantage of refining the treatedpine to lower freeness, such as 400 CSF (depending on drainage orfurnish mix requirements on paper machines).

In terms of mechanical properties, the treatment impacted significantlythe Tear strength, reducing it to the level of hardwood (FIG. 8). Thisis acceptable when using the treated pine fibers to replace hardwoodfibers in a paper furnish. The reduction in Tear results fromsignificant fiber length reduction, and the effect of chemistry.

Other mechanical properties were only slightly affected, and remainsubstantially higher than hardwood furnish. Interestingly, as shown inTable II, the elastic stiffness of treated pine can even be higher thanthat of the control pine.

EXAMPLE 4

The treated pine as in Example 3 above, refined to 560 CSF, was alsomixed with hardwood pulp of a range of freeness, to investigate themixed furnish paper properties such as bulk and smoothness. The resultsare listed in Table III.

TABLE III Sheffield Smoothness Bulk, cc/g 10% Treated Pine (560 323 1.83CSF) + 308 1.83 90% Hardwood 171.2 1.37 137.8 1.33 20% Treated Pine (560302 1.75 CSF) + 231.8 1.5 80% Hardwood 182.8 1.43 136.6 1.32 50% TreatedPine (560 318 1.79 CSF) + 182.4 1.41 50% Hardwood 163.4 1.38 147.6 1.29

FIG. 9 plots the bulk-smoothness curve of the mixed pulp furnish (datafrom Table III), along with 100% pine and hardwood curves (data fromTable II). It is obvious that the treated pine can be used to replacesubstantial amounts of hardwood pulp. The exact amount of hardwoodreplacement in the paper mill, however, may also be affected somewhat bythe nature, type and optimization of commercial refiners.

EXAMPLE 5

A Voith LR1 Disc Refiner was used to refine bleached southern pine whichhad been treated with 1% hydrogen peroxide, as catalyzed by Fe(III) atpH4. The refiner specific edge load was set at 0.8 Ws/m. As seen fromTable IV, FIG. 10, energy saving and fiber length reduction wereconfirmed.

TABLE IV Treated Southern Pine Control Southern Pine Kajaani KajaaniRefining average average Energy, fiber length, fiber length, kW · h/tonpulp Freeness L(L) Freeness L(L) 0 750 CSF 2.07 mm 750 CSF 2.11 mm 46677 CSF 2.05 mm 722 CSF 2.12 mm 78 610 CSF 1.98 mm 677 CSF 2.12 mm 118455 CSF 1.84 mm 633 CSF 2.14 mm 158 317 CSF 1.66 mm 579 CSF 2.09 mm 198197 CSF 1.48 mm 538 CSF 2.10 mm

EXAMPLE 6

A Voith LR 1 Disc Refiner was used to refine bleached southern pine,which had been treated with 1% hydrogen peroxide, as catalyzed by Fe(II)at pH4. The refiner specific edge load was set at 4 km.

From Table V, FIGS. 11, 12, it is seen that energy saving and fiberlength reduction were confirmed.

TABLE V Treated Southern Pine Refining 25 46 99 119 — Energy, kW · h/tonFreeness  590 CSF  442 CSF 185 CSF 115 CSF — Kajaani  1.9 mm 1.72 mm 1.4 mm  1.2 mm — average length L(L) Untreated Pine - Control Refining 0 29 40  75 90 Energy, KW · h/ton Freeness  730 CSF  671 CSF 657 CSF —522 CSF Kajaani 2.14 mm — —  2.12  1.93 average length L(L)

What is claimed:
 1. A method for modulating the morphology of cellulosic fibers comprising the steps of subjecting the fibers to a metal ion-activated peroxide treatment carried out at a pH of between about 1 and about 9 and subjecting the treated fibers to a refining treatment.
 2. The method of claim 1 wherein said metal ion is a transitional metal ion.
 3. The method of claim 1 wherein said metal ion is iron.
 4. The method of claim 1 wherein said pH is between about 3 and about
 7. 5. The method of claim 1 wherein the fibers are subjected to the solution at temperatures between about 40 degrees C. to about 120 degrees C.
 6. The method of claim 1 wherein the fibers are subjected to the solution for between about 10 minutes to about 10 hour.
 7. The method of claim 1 wherein said peroxide is present with said solution at a concentration of between about 0.2% and about 5% based on pulp.
 8. The method of claim 1 wherein said metal ion is present in said solution at a concentration of between about 0.002% and about 0.1% on pulp.
 9. The method of claim 1 wherein said pulp is subjected to said solution for a time sufficient to substantially act on at least the cellulose and hemi-cellulose of the pulp, causing oxidation and oxidative degradation of cellulose fibers.
 10. A softwood pulp having a modified morphology, leading to paper making properties substantially functionally equivalent to hardwood pulp papermaking properties.
 11. The softwood pulp of claim 10 wherein the fibers of said softwood pulp, after treatment, exhibit a substantially shorter fiber length and distribution, and enhanced fiber collapsibility, than prior to treatment.
 12. The softwood pulp of claim 9 wherein said pulp is oxidatively degraded relative to untreated softwood pulp.
 13. The softwood pulp of claim 10 wherein the pulp exhibits a Canadian Standard Freeness of between about 115 and about
 590. 14. The softwood pulp of claim 13 wherein the pulp exhibits a Kajaani average fiber length of between about 1.0 and 1.9 mm.
 15. A pulp comprising between about 50% and 90% hardwood pulp and the remainder being softwood pulp which has been subjected to a metal ion-activated peroxide treatment carried out at a pH of between about 2 and about 9 and a refining treatment.
 16. The pulp of claim 15 wherein said metal ion is a transitional metal.
 17. The pulp of claim 15 wherein said metal ion is iron and said pH is between about 3 and about
 7. 18. The pulp of claim 15 wherein said pulp is substantially functionally equivalent to a hardwood pulp as respects the usefulness of the pulp in papermaking.
 19. The softwood pulp of claim 11 wherein the pulp is used to manufacture a paper web material. 